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Abstract:

A display image to be projected onto a projection surface is formed on an
imaging surface of a screen member at time of projecting the display
image onto the projection surface. A projector is adapted to project a
light, which forms the display image on the imaging surface. The imaging
surface is formed as a convex surface that limits a curvature of field of
the virtual image.

Claims:

1. A head-up display apparatus, which is adapted to project a display
image onto a projection surface of a display member to enable a viewer to
view a virtual image of the display image from a predetermined viewing
area, the head-up display apparatus comprising: a screen member that
includes an imaging surface, on which the display image to be projected
onto the projection surface is formed at time of projecting the display
image onto the projection surface; and a projector that is adapted to
project a light, which forms the display image on the imaging surface,
wherein the imaging surface is formed as a convex surface that limits a
curvature of field of the virtual image.

2. The head-up display apparatus according to claim 1, wherein: the
projection surface is formed as a concave surface; and the imaging
surface is formed as the convex surface that limits the curvature of
field of the virtual image, which is caused by a curvature of the
projection surface.

3. The head-up display apparatus according to claim 1, wherein the
projection surface is formed as a concave surface, which is curved and is
recessed in a direction away from the predetermined viewing area.

4. The head-up display apparatus according to claim 1, wherein the
imaging surface is curved and is protruded toward the projector.

5. The head-up display apparatus according to claim 1, wherein the
imaging surface is three-dimensionally curved in both of a first
direction and a second direction, which are perpendicular to each other,
based on a curvature of the projection surface in the first direction and
a curvature of the projection surface in the second direction.

6. The head-up display apparatus according to claim 1, further comprising
a magnifying mirror that includes a reflection surface, which is formed
as a concave surface and projects the display image onto the projection
surface at time of projecting the display image onto the projection
surface by reflecting the display image formed on the imaging surface
upon enlarging the display image by the reflection surface, wherein the
imaging surface is formed as the convex surface that limits the curvature
of field of the virtual image, which is caused by a curvature of the
reflection surface.

7. The head-up display apparatus according to claim 6, wherein the
reflection surface is curved and is recessed in a direction away from the
imaging surface.

8. The head-up display apparatus according to claim 6, wherein the
reflection surface is three-dimensionally curved in both of a first
direction and a second direction, which are perpendicular to each other,
based on a curvature of the projection surface in the first direction and
a curvature of the projection surface in the second direction.

9. The head-up display apparatus according to claim 6, wherein: the
display image projected onto the display member is elongated in a first
direction rather than a second direction of the display image, which is
perpendicular to the first direction; the reflection surface is formed as
the concave surface, which enlarges the display image projected onto the
display member such that a magnification of the display image in the
first direction is larger than a magnification of the display image in
the second direction; and the imaging surface is formed as the convex
surface, which is more curved in the first direction than the second
direction to limit the curvature of field of the virtual image in the
first direction.

10. The head-up display apparatus according to claim 1, further
comprising a focusing point adjusting optical system that adjusts a
focusing point, in which the light projected from the projector is
focused, to place the focusing point on the imaging surface.

11. The head-up display apparatus according to claim 10, wherein the
focusing point adjusting optical system includes a free-form-surface
optical element that has one of a light input surface and a light output
surface, which has a curved surface section that is defined by a
polynomial having an odd-order term.

12. The head-up display apparatus according to claim 1, wherein: the
projector is adapted to project a laser light as the light; and the
projector scans the laser light on the imaging surface to form the
display image on the imaging surface.

13. The head-up display apparatus according to claim 1, wherein: the
head-up display apparatus is for a vehicle; and the projection surface of
the display member is one of: a surface of a windshield of the vehicle;
and a surface of a separate member that is formed separately from the
windshield and is placed adjacent to an interior surface of the
windshield.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on and incorporates herein by reference
Japanese Patent Application No. 2011-161466 filed on Jul. 24, 2011.

TECHNICAL FIELD

[0002] The present disclosure relates to a head-up display apparatus.

BACKGROUND

[0003] In a field of a head-up display apparatus of a vehicle, it is known
that aberrations, which are generated in an optical system including a
windshield of the vehicle, cause deformation of an image, which is
displayed as a virtual image. One of the aberrations of the optical
system is an aberration caused by a distortion of the image. A technique
of reducing a two-dimensional deformation of the virtual image, which is
caused by the distortion, is disclosed in, for example, JPH07-257225A,
JPH10-149085A and JPH11-30764A.

[0004] Specifically, JPH07-257225A teaches a holographic display system,
which includes a light emitting display means and a reflective hologram.
The light emitting display means projects a light of a virtual image. The
light, which is outputted from the light emitting display means, is
projected onto the reflective hologram. A shape of the display image,
which is projected by the light emitting display means, is pre-deformed
(pre-distorted) in advance to compensate the deformation generated at the
reflective hologram. Thus, it is possible to limit the two-dimensional
deformation of the displayed virtual image.

[0005] JPH10-149085A teaches a holographic display apparatus, which
includes a display, a light source and a hologram combiner. The display
projects a light, which forms a display image. The light, which is
outputted from the display, is projected onto the hologram combiner. A
shape of the display image, which is projected by the display, is
pre-deformed (pre-distorted) in advance to compensate the two-dimensional
deformation of the image generated at the hologram combiner. Thus, it is
possible to limit the two-dimensional deformation of the displayed image.

[0006] JPH11-30764A teaches a head-up display apparatus, which includes an
image display surface and a half mirror. A light of an image is projected
from the image display surface. The light, which is outputted from the
image display surface, is projected onto the half mirror. The image,
which is projected from the image display surface, is pre-deformed
(pre-distorted) in advance to compensate a deformation of the image
generated at the half mirror. Thus, it is possible to limit the
two-dimensional deformation of the virtual image.

[0007] Lately, like in the case of JPH11-30764A, it is popular to use the
head-up display apparatus, which projects a display image onto a concave
windshield located on a front side of a viewer. In such a head-up display
apparatus, in addition to the aberration caused by the two-dimensional
deformation of the image, an aberration caused by a three-dimensional
curvature of field occurs. Therefore, the displayed virtual image of the
display image, which is viewed at a viewing area by a viewer, is deformed
such that a distance between the viewing area of the viewer and the
displayed virtual image decreases from a center portion of the displayed
virtual image to an edge portion of the displayed virtual image.

[0008] In the head-up display apparatus of JPH11-30764A, the image display
surface, which displays the display image, is formed as a planar surface.
In addition, in general, it is difficult to change the shape of such an
image display surface. Therefore, it is difficult to adjust a distance of
a light path, which is from the image display surface to the windshield,
and an imaging point of the virtual image. As a result, the
three-dimensional deformation of the virtual image cannot be reduced. In
the case where the three-dimensional deformation is generated in the
virtual image, when the viewer moves his/her view point within the
viewing area, a change in the shape and a change in the position occur in
the virtual image of the display image. Therefore, the display quality of
the display image, which is displayed as the virtual image, may possibly
become insufficient.

SUMMARY

[0009] The present disclosure is made in view of the above disadvantages.

[0010] According to the present disclosure, there is provided a head-up
display apparatus, which is adapted to project a display image onto a
projection surface of a display member to enable a viewer to view a
virtual image of the display image from a predetermined viewing area. The
head-up display apparatus includes a screen member and a projector. The
screen member includes an imaging surface, on which the display image to
be projected onto the projection surface is formed at time of projecting
the display image onto the projection surface. The projector is adapted
to project a light, which forms the display image on the imaging surface.
The imaging surface is formed as a convex surface that limits a curvature
of field of the virtual image.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The drawings described herein are for illustration purposes only
and are not intended to limit the scope of the present disclosure in any
way.

[0012]FIG. 1 is a schematic view showing a vehicle, in which a head-up
display apparatus of an embodiment of the present disclosure is
installed;

[0013]FIG. 2 is a diagram showing an arrangement of the head-up display
apparatus of the embodiment in the vehicle;

[0014]FIG. 3 is a diagram showing a structure of a laser scanner of the
head-up display apparatus of the embodiment;

[0015]FIG. 4 is a diagram showing locations of the components of the
head-up display apparatus of the embodiment;

[0016]FIG. 5 is a diagram showing a convex imaging surface of a screen of
the head-up display apparatus of the present embodiment along with a
windshield of the vehicle and a virtual image viewed in a direction of an
arrow V in FIG. 2;

[0017]FIG. 6 is a diagram showing a free-form-surface lens along with the
laser scanner and the imaging surface of the screen of the head-up
display apparatus of the present embodiment;

[0018]FIG. 7A is a diagram showing a display image formed on the imaging
surface;

[0019]FIG. 7B is a diagram showing a shape of a virtual image in a case
where the imaging surface is a planar surface;

[0020]FIG. 7C is a diagram showing the virtual image of FIG. 7B, which is
viewed by a viewer when an eye point of the viewer is moved in a right
direction;

[0021]FIG. 7D is a diagram showing a shape of the virtual image in a case
where the imaging surface is a convex surface;

[0022]FIG. 7E is a diagram showing the virtual image of FIG. 7D, which is
viewed by the viewer when the eye point of the viewer is moved in the
right direction;

[0023]FIG. 8 is a diagram showing a specification of an apparatus having
a convex imaging surface of the embodiment;

[0024]FIG. 9 is a diagram showing a specification of an apparatus having
a planar imaging surface in a comparative example;

[0025] FIG. 10 is a diagram showing spot diameters of laser lights
measured in a simulation using the apparatus of FIG. 8, which has the
convex imaging surface, and spot diameters of laser lights measured in a
simulation using the apparatus of FIG. 9, which has the planar imaging
surface;

[0026] FIG. 11 is a diagram showing a specification of an apparatus having
a free-form-surface lens of the present embodiment; and

[0027] FIG. 12 is a diagram showing spot diameters of laser lights
measured in a simulation using the apparatus of FIG. 11, which has the
free-form-surface lens of the embodiment, and spot diameters of laser
lights measured in a simulation using an apparatus, which does not have
the free-form-surface lens.

DETAILED DESCRIPTION

[0028] An embodiment of the present disclosure will be described with
reference to the accompanying drawings.

[0029] With reference to FIGS. 1 to 3, a head-up display apparatus 100 of
an embodiment of the present disclosure is received in, for example, an
instrument panel of a vehicle (an automobile in this instance) 1. A
display image 71 is projected from the head-up display apparatus 100 onto
a windshield (a display member) 90 of the vehicle 1, so that a driver
(viewer) can visually recognize a virtual image 70 of the display image
71 from a predetermined eye box 60. A projection surface (also referred
to as a surface of projection) 91, onto which the display image 71 is
projected from the head-up display apparatus 100, is formed in a vehicle
interior side surface of the windshield 90 and is formed as a concave
surface, which is concave, i.e., is curved and is recessed in a direction
away from the viewer (and thereby away from the eye box 60 of the
viewer). The light of the display image 71, which is projected onto the
projection surface 91, is reflected by the projection surface 91 toward
the eye box 60 and reaches an eye point 61 of the driver (the viewer).
The viewer who perceives the light of the display image 71 can visually
recognize, i.e., can view the virtual image 70 of the display image 71,
which is formed on the front side of the windshield 90 (i.e., the side of
the windshield 90, which is opposite from the viewer).

[0030] The display image 71, which is projected onto the projection
surface 91, is configured into an elongated rectangular form (an oblong
form) that has a horizontal length, which is measured in a horizontal
direction (a left-to-right direction) of the vehicle and is larger than a
vertical length of the display image 71 measured in a vertical direction
of the vehicle. This is because of that the movement of the eye point 61
is easier in the horizontal direction than the vertical direction when
the viewer is seated on his/her seat in the vehicle. The display image 71
includes image segments indicating, for example, a traveling speed of the
vehicle having the head-up display apparatus 100, an image of a traveling
direction sign of the vehicle, which is specified by the navigation
system, and a warning sign(s) of the vehicle.

[0031] Now, a structure of the head-up display apparatus 100 will be
described with reference to FIGS. 2 and 3. The head-up display apparatus
100 includes a laser scanner 10, a screen 30 and a concave mirror 40,
which are received in a housing 110 (FIG. 1). In the following
description, an axis of the horizontal direction (also referred to as a
lateral direction or a first direction) of the virtual image 70, which is
viewed by the viewer, will be referred to as an x-axis. Furthermore, an
axis of the vertical direction (also referred to as a top-to-bottom
direction or a second direction) of the virtual image 70, which is
perpendicular to the horizontal direction, will be referred to as a
y-axis. Also, in the following description, for the sake of convenience,
the direction of the x-axis of the display image 71, which is formed on
or projected onto each corresponding component, will be referred to as
the horizontal direction (also referred to as the lateral direction or
the first direction), and the direction of the y-axis of the display
image 71, which is formed on or projected onto each corresponding
component, will be referred to as the vertical direction (also referred
to as the top-to-bottom direction or the second direction).

[0033] The light source 13 includes three laser projecting devices 14-16.
Each of the laser projecting devices 14-16 projects a corresponding laser
light (also referred to as a laser beam) that has a frequency, which is
different from that of the other two of the laser projecting devices
14-16, i.e., the corresponding laser light that has a color phase, which
is different from that of the other two of the laser projecting devices
14-16. Specifically, the laser projecting device 14 projects the laser
light of a red color. The laser projecting device 15 projects the laser
light of a blue color. The laser projecting device 16 projects the laser
light of a green color. When the laser lights of the different color
phases are additively mixed, various colors can be reproduced. Each laser
projecting device 14-16 is connected to the controller 11. Each laser
projecting device 14-16 projects the laser light of the corresponding
color phase based on a control signal outputted from the controller 11.

[0034] The optical device 20 includes three collimator lenses 21, three
dichroic filters 22-24 and a condenser lens 25. Each collimator lens 21
is placed on a downstream side of the corresponding laser projecting
device 14-16 in the projecting direction of the laser light, which is
projected from the laser projecting device 14-16. The collimator lens 21
generates parallel rays of light by bending the laser light.

[0035] Each dichroic filter 22-24 is placed on a downstream side of the
corresponding collimator lens 21 in the projecting direction of the laser
light, which is projected from the corresponding laser projecting device
14-16. Each dichroic filter 22-24 reflects a light of a predetermined
corresponding frequency and passes lights of other frequencies, which are
other than the predetermined corresponding frequency. Specifically, the
dichroic filter 22, which is placed on the downstream side of the laser
projecting device 14, reflects the light of the frequency of the red
color and passes the other lights of the other frequencies that are other
than the frequency of the red color. The dichroic filter 23, which is
placed on the downstream side of the laser projecting device 15, reflects
the light of the frequency of the blue color and passes the other lights
of the other frequencies that are other than the frequency of the blue
color. The dichroic filter 24, which is placed on the downstream side of
the laser projecting device 16, reflects the light of the frequency of
the green color and passes the other lights of the other frequencies that
are other than the frequency of the green color. Each dichroic filter
22-24 reflects the corresponding laser light toward the condenser lens
25.

[0036] The condenser lens 25 is a plano-convex lens, which has a light
input surface formed as a planar surface and a light output surface
formed as a convex surface. The condenser lens 25 converges the light by
bending the laser light, which enters the light input surface of the
condenser lens 25. Thereby, the laser light, which has passed through the
condenser lens 25, is focused on an imaging surface 31 of the screen 30,
which will be described later.

[0037] The MEMS mirror 26 is connected to the controller 11 and is
configured generally into a rectangular plate form. The MEMS mirror 26
includes an outer frame portion 29, an inner frame portion 28 and a
mirror portion 27.

[0038] The outer frame portion 29 is configured into a rectangular frame
form, which surrounds an outer peripheral part of the inner frame portion
28 and an outer peripheral part of the mirror portion 27. The outer frame
portion 29 is securely held by the housing of the laser scanner 10. The
inner frame portion 28 is placed on an inner side of the outer frame
portion 29 and is configured into a rectangular frame form. The inner
frame portion 28 is supported by the outer frame portion 29 through two
low speed pivots 28a, which extend in the horizontal direction. The inner
frame portion 28 is rotatable (pivotable, i.e., swingable) about the low
speed pivots 28a (more specifically, about the axis of the low speed
pivots 28a). A plurality of undepicted electrodes (a group of electrodes)
is provided between the outer frame portion 29 and the inner frame
portion 28 to rotate the inner frame portion 28 about the low speed
pivots 28a.

[0039] The mirror portion 27 is placed at the inner side of the inner
frame portion 28 and is configured into circular disk form. A metal film
is formed on a surface of the mirror portion 27, which is opposed to the
optical device 20, by vapor deposition of, for example, aluminum to
reflect the light at a high efficiency. The mirror portion 27 is
supported by the inner frame portion 28 by two high speed pivots 27a,
each of which extends in the vertical direction. The mirror portion 27 is
rotatable (pivotable, i.e., swingable) about the high speed pivots 27a
(more specifically, about the axis of the high speed pivots 27a). A
plurality of undepicted electrodes (a group of electrodes) is provided
between the inner frame portion 28 and the mirror portion 27 to rotate
the mirror portion 27 about the high speed pivots 27a. In the MEMS mirror
26, which is constructed in the above-described manner, the group of
electrodes provided between the outer frame portion 29 and the inner
frame portion 28 and the group of electrodes provided between the inner
frame portion 28 and the mirror portion 27 are driven based on a drive
signal that is outputted from the controller 11. Thereby, the orientation
of the mirror portion 27 can be adjusted in the vertical direction (see a
direction VS in FIG. 3) and the horizontal direction (see a direction HS
in FIG. 3).

[0040] The controller 11 is an electronic control device, which includes a
processor and is connected to the laser projecting devices 14-16 and the
MEMS mirror 26. The controller 11 outputs the control signal to each
laser projecting device 14-16 to blink the laser light like a pulsed
light. In addition, the laser controller 11 outputs the drive signal to
the MEMS mirror 26 to control the direction of the reflected laser light,
which is reflected by the mirror portion 27, such that the reflected
laser light, which is reflected by the mirror portion 27, forms a
scanning line SL shown in FIG. 3.

[0041] The controller 11 controls the laser scanner 10 such that the laser
scanner 10 projects the light, which forms the display image 71 on the
imaging surface 31 of the screen 30. Specifically, by scanning the
projected blinking laser light, the display image 71, each pixel of which
is formed by the corresponding blinking laser light, is formed on the
imaging surface 31 of the screen 30. The display image 71, which is
formed by the scanning of the laser scanner 10, is an image that has, for
example, 60 frames per second and 480 pixels in the horizontal direction
(the x-axis) and 240 pixels in the vertical direction (the y-axis).

[0042] The screen 30 is a reflective screen (a screen of a reflective
type), which is formed by, vapor deposition of, for example, aluminum on
a surface of a substrate made of, for example, glass. The screen 30 is
placed on the upper side of the laser scanner 10 in the vertical
direction of the vehicle (see FIG. 4). The screen 30 has the imaging
surface 31. The imaging surface 31 is formed by a metal film of, for
example, aluminum that is vapor deposited on the screen 30. The display
image 71 is formed on the imaging surface 31 when the laser light is
projected from the laser scanner 10 along a y-z plane, which is defined
by the y-axis and a z-axis (see FIG. 4). The z-axis is perpendicular to
both of the x-axis and the y-axis. The imaging surface 31 has
micro-asperities to diffuse the laser light. The imaging surface 31
diffuses and reflects the laser light, which forms the display image 71
and impinges on the imaging surface 31, toward the concave mirror 40.

[0043] The concave mirror 40 is formed through vapor deposition of, for
example, aluminum on a surface of a substrate made of, for example,
glass. The concave mirror 40 has a reflection surface 41 that reflects
the reflected laser light, which is reflected from the imaging surface 31
of the screen 30, onto the projection surface 91 of the windshield 90. A
center portion of the reflection surface 41 is concave, i.e., is curved
and is recessed in a direction away from the imaging surface 31 and the
projection surface 91. The reflection surface 41 projects the display
image 71 on the projection surface 91 such that the reflection surface 41
enlarges and reflects the display image 71, which is reflected by the
imaging surface 31. The magnification of the display image 71, which is
magnified by the curvature of the reflection surface 41, differs between
the horizontal direction and the vertical direction of the display image
71. Specifically, the curvature of the reflection surface 41 in the
horizontal direction is larger than the curvature of the reflection
surface 41 in the vertical direction, so that the magnification
(magnification scale) of the display image 71 in the horizontal direction
is larger than the magnification (magnification scale) of the display
image 71 in the vertical direction on the reflection surface 41.

[0044] Next, the characteristic features of the head-up display apparatus
100 of the embodiment will be described. As shown in FIGS. 2 and 4, the
imaging surface 31 of the screen 30 is a curved convex surface, which is
convex, i.e., is curved and is protruded toward the laser scanner 10 and
the concave mirror 40. In addition, a free-form-surface lens (serving as
a free-form-surface optical element) 50 is placed between the laser
scanner 10 and the screen 30. Now, the imaging surface 31 and the
free-form-surface lens 50 will be described in detail with reference to
FIGS. 4 to 7E.

[0045] As shown in FIGS. 4 and 5, the imaging surface 31 of the screen 30
is protruded toward the concave mirror 40 and is curved in the horizontal
direction of the display image 71, which is formed on the imaging surface
31. Specifically, the imaging surface 31 of the screen 30 is convex,
i.e., is protruded and is curved such that a center portion 32 of the
imaging surface 31 is closer toward a side, in which the reflection
surface 41 of the concave mirror 40 (and also the projection surface 91)
is located, in comparison to an edge portion 33 of the imaging surface 31
in a light transmission direction (the direction of the z-axis) of the
laser light. The shape of the imaging surface 31 is chosen to compensate
(correct or limit) a curvature of field of the virtual image 70, which is
caused by the curvature of the reflection surface 41 and the curvature of
the projection surface 91. Here, it should be noted that the curvature of
the imaging surface 31, the curvature of the projection surface 91 and
the curvature of the virtual image 70 shown in the drawings are not in
scale and are slightly exaggerated for the descriptive purpose.

[0046] Now, a three-dimensional deformation of the virtual image 70, which
is caused by the curvature of field, will be described. The laser light,
which is reflected by the imaging surface 31, is further reflected by the
curved reflection surface 41 and the curved projection surface 91. Due to
these reflections, an aberration is generated on the virtual image 70 by
the curvature of field. Specifically, with reference to FIG. 5, the laser
light, which is reflected by the edge portion 33 of the imaging surface
31, is imaged as the corresponding part (an edge portion 70b) of the
virtual image 70 at a closer location that is closer to the windshield 90
in comparison to the laser light, which is reflected by the center
portion 32 of the imaging surface 31. Therefore, in a case where the
imaging surface is a planar surface, the virtual image 70 of the display
image 71, which is visually recognized by the viewer, is curved such that
the distance between the planar imaging surface (see the imaging surface
31 indicated by a dot-dot-dash line in FIG. 5) and the virtual image 70
is progressively reduced from a center portion 70a of the virtual image
70 to the edge portion 70b of the virtual image 70 (see a dot-dot-dash
line in FIG. 5).

[0047] In view of the above point, according to the present embodiment,
the imaging surface 31 is formed into the curved convex surface, which is
three-dimensionally configured and compensates (corrects or limits) the
curvature of field of the virtual image 70. Because of this shape of the
imaging surface 31, a distance from the imaging surface 31 to the
projection surface 91 in the light transmission direction of the laser
light increases from the center portion 32 of the imaging surface 31 to
the edge portion 33 of the imaging surface 31. Therefore, the imaging
surface 31 has the adjusting function for adjusting the imaging location
such that the imaging location of the edge portion 70b of the virtual
image 70 is displaced away from the imaging surface 31 in the greater
amount in comparison to that of the center portion 70a of the virtual
image 70. The edge portion 70b of the virtual image 70, which is placed
close to the projection surface 91 due to the curvature of field, is now
further spaced from the projection surface 91 because of this adjusting
function of the imaging surface 31. Thereby, the three-dimensional
deformation of the virtual image 70 is reduced.

[0048] The effect of the imaging surface 31 on the virtual image 70 will
be described in detail with reference to a result of a specific
simulation, which is performed with the apparatus having the
specification shown in FIG. 8. FIG. 8 shows the specification of the
apparatus having the convex imaging surface 31. The imaging surface 31,
which is indicated in FIG. 8, is a surface (quadric surface, more
specifically a parabolic surface) that has a quadratic term in the
horizontal direction (the x-axis). Therefore, the imaging surface 31 is
curved parabolically in the horizontal direction. Furthermore, FIG. 9
indicates the specification of the comparative apparatus having the
planar imaging surface. A spot diameter of the laser light of the virtual
image 70, which is viewed from the eye point 61 in the eye box 60, is
compared in FIG. 10 for the specification of FIG. 8 and the specification
of FIG. 9. The spot diameter is a diameter of the laser light in a plane
that is perpendicular to the light transmission direction of the laser
light. When the spot diameter is reduced, the virtual image 70 is less
moved in response to the positional change of the eye point 61 and
thereby becomes clear.

[0049] As indicated in FIG. 10, when the convex imaging surface (convex
surface) 31 is used, the spot diameter of the laser light in the center
portion of the display image 71 is reduced. In addition, the maximum spot
diameter of the laser light, which is maximum throughout the entire range
of the display image 71, is also reduced. Therefore, the virtual image 70
is less moved in response to the movement of the eye point 61 and thereby
becomes clear.

[0050] Next, the function of the free-form-surface lens 50 will be
described. As shown in FIG. 6, the focusing point of the laser light,
which is projected by the laser scanner 10, is located along a concentric
spherical surface SF (see a dotted line in FIG. 6), which is centered at
the center of the laser scanner 10 (specifically, the center of the
mirror portion 27 of the MEMS mirror 26). Therefore, at the time of
executing the horizontal scanning of the laser light (see an arrow HS in
FIG. 6), the focusing point of the laser light may substantially deviate
from the imaging surface 31 depending on the shape of the convex imaging
surface 31. Specifically, in a case where the focusing points of the
laser lights are set to place the focusing point of the laser light onto
the surface section of the center portion 32 of the imaging surface 31,
the focusing point of the laser light in the area of the edge portion 33
may deviate from the imaging surface 31 on a side where the laser scanner
10 is located (see an arrow G1 in FIG. 6). In contrast, in another case
where the focusing points of the laser lights are set to place the
focusing point of the laser light onto the surface section of the edge
portion 33, the focusing point of the laser light in the area of the
center portion 32 may deviate from the imaging surface 31 on an opposite
side, which is opposite from the laser scanner 10 (see an arrow G2 in
FIG. 6). Thereby, the display image 71, which is formed on the imaging
surface 31, may possibly become unclear.

[0051] In view of the above point, the free-form-surface lens 50 is placed
between the laser scanner 10 and the imaging surface 31 according to the
embodiment. The free-form-surface lens 50 is a lens, which has a light
input surface 51 and a light output surface 52 and is made of optical
glass. The light input surface 51 is formed as a free-form surface and is
opposed to the laser scanner 10 in the light transmission direction of
the laser light, i.e., is placed on the side where the laser scanner 10
is located. The light output surface 52 is formed as a planar surface.
The free-form-surface lens 50 adjusts the focusing point of the laser
light, which is projected by the laser scanner 10 onto the imaging
surface 31. Specifically, the light input surface 51 of the
free-form-surface lens 50 is recessed in the direction away from the
laser scanner 10 and is curved in the horizontal direction. Therefore,
the focusing point of the laser light, which forms a portion of the
display image 71 at the horizontal edge portion 33 (i.e., the horizontal
edge portion 33 that is located at the end of the display image 71 in the
horizontal direction of the imaging surface 31), is displaced further
away from the laser scanner 10. Thereby, even when the focusing point of
the laser light is set to place the focusing point of the laser light
onto the surface section of the center portion 32, the focusing point of
the laser light in the area of the horizontal edge portion 33 of the
display image 71 can be placed onto the imaging surface 31 by the
free-form-surface lens 50. Because of the above function of the
free-form-surface lens 50, the focusing point of the laser light can be
substantially placed on the imaging surface 31 throughout the entire
range of the imaging surface 31. Thereby, the display image 71 can be
clearly formed throughout the entire range of the imaging surface 31. As
a result, the virtual image 70 (see FIG. 5) of the display image 71,
which is viewed by the viewer, becomes a more clear image throughout the
entire range of the virtual image 70.

[0052] The effect of the free-form-surface lens 50 on the virtual image 70
will now be described in detail with reference to a result of a specific
simulation, which is performed with the apparatus having the
specification shown in FIG. 11. FIG. 11 shows the specification of the
apparatus having the free-form-surface lens 50. The light input surface
51 of the free-form-surface lens 50, which is indicated in FIG. 11, has a
quadratic term and a quartic term in the horizontal direction (the
direction of the x-axis). In addition, the light input surface 51 has a
quadratic term, a cubic term and a quartic term in the vertical direction
(the direction of the y-axis).

[0053] In the free-form-surface lens 50 shown in FIG. 11, because of the
even-order terms in the horizontal direction, the free-form-surface lens
50 is concave, i.e., is recessed and is curved in the horizontal
direction. The focusing point of the laser light is adjusted onto the
curved imaging surface 31. Furthermore, the free-form-surface lens 50 has
the cubic term, which is the odd-order term, so that the lower half of
the light input surface 51 in the vertical direction is convex, i.e., is
curved and is protruded toward the laser scanner 10. In contrast, an
upper half of the light input surface 51 in the vertical direction is
concave, i.e., is curved and is recessed in a direction away from the
laser scanner 10. Because of the above-described configuration of the
light input surface 51, the laser light, which passes through the lower
half of the light input surface 51 and reaches the lower half of the
imaging surface 31, is focused at a location, which is closer to the
laser scanner 10 in comparison to the case where the free-form-surface
lens 50 is eliminated. In contrast, the laser light, which passes through
the upper half of the light input surface 51 and reaches the upper half
of the imaging surface 31, is focused at a location, which is further
away from the laser scanner 10 in comparison to the case where the
free-form-surface lens 50 is eliminated.

[0054] In the present embodiment, the laser scanner 10 is placed at the
lower side of the screen 30. Therefore, the distance from the laser
scanner 10 to the screen 30 is increased toward the upper side in the
vertical direction. Thus, since the focusing point of the laser light is
adjusted by the free-form-surface lens 50, the focusing point of the
laser light can be adjusted onto the imaging surface 31 throughout the
entire range of the imaging surface 31 even in the case where the laser
light is projected from the lower side along the y-z plane. FIG. 12 shows
the comparison between the case where the free-form-surface lens 50
having the above function is provided and the case where the
free-form-surface lens 50 is eliminated.

[0055] As indicated in FIG. 12, when the free-form-surface lens 50 is
placed between the laser scanner 10 and the screen 30, the spot diameter
of the laser light at the center portion of the display image 71 located
in the center portion 32 of the screen 30 is slightly increased when the
free-form-surface 50 lens is provided. Here, it should be noted that the
indication of "0.0 μm" in FIG. 12 means that the spot diameter of the
laser light in the center portion of the display image 71 in the absence
of the free-form-surface lens is smaller than 0.1 μm but is larger
than zero. The maximum spot diameter of the laser light in the entire
range of the display image 71 is decreased when the free-form-surface
lens 50 is provided. Therefore, the virtual image 70 is less moved in
response to the movement of the eye point 61 and thereby becomes more
clear throughout the entire range of the virtual image 70 in the presence
of the free-form-surface lens 50.

[0056] With reference to FIGS. 7A to 7E, the change of the virtual image
70 will be described for the case where the viewer moves the eye point 61
within the eye box 60. FIG. 7A shows the shape of the display image 71,
which is configured generally into the elongated rectangular form (an
oblong form) elongated in the direction of the axis x and is formed on
the imaging surface 31. In the case where the imaging surface is the
planar surface, the virtual image 70, which is viewed by the viewer, has
a shape shown in FIG. 7B. The shape of this virtual image 70 is curved in
the horizontal direction (see the virtual image 70 indicated by a
dot-dot-dash line in FIGS. 2 and 5). Therefore, in the case where the
viewer moves the eye point 61 in, for example, the right direction, the
virtual image 70 has the shape shown in FIG. 7C. Specifically, the right
half of the virtual image 70 is compressed in the horizontal direction,
and the left half of the virtual image 70 is expanded in the horizontal
direction. In addition, since the virtual image 70 is curved, a distance
between the virtual image 70 and the eye point 61 is changed in response
to the movement of the eye point 61 in the horizontal direction. Thus,
when the viewer moves the eye point 61, the virtual image 70 approaches
the viewer. As discussed above, the shape and the location of the virtual
image 70 are significantly changed in response to the movement of the eye
point 61.

[0057]FIG. 7D shows the virtual image 70, which is formed on the convex
imaging surface 31. In the virtual image 70, which is shown in FIG. 7D,
the curvature of the virtual image 70 in the horizontal direction is
reduced (see the virtual image 70 indicated by a solid line in FIGS. 2
and 5). In the present embodiment, in the case where the viewer moves the
eye point 61 in the right direction, the virtual image 70 has the shape
shown in FIG. 7E. In the case of FIG. 7E, the compression of the right
half of the virtual image 70 of FIG. 7E, which is located on the right
side in the horizontal direction, and the expansion of the left half of
the virtual image 70 of FIG. 7E, which is located on the left side in the
horizontal direction, are reduced in comparison to the virtual image 70
of FIG. 7C. In addition, since the curvature of the virtual image 70 is
reduced, the change in the distance between the virtual image 70 and the
eye point 61 caused by the movement of the eye point 61 in the horizontal
direction is more limited.

[0058] As discussed above, in the present embodiment, even when the viewer
moves the eye point 61 within the eye box 60, the change in the shape of
the virtual image 70 and the change in the position of the virtual image
70 are both limited. Therefore, the display quality of the display image
71, which is displayed as the virtual image 70, can be improved.

[0059] Furthermore, even in the present embodiment, in which the imaging
surface 31 is configured to be convex to reduce the three-dimensional
deformation of the virtual image 70, the free-form-surface lens 50 can
adjust the focusing point of the laser light, and thereby the loss of the
clearness of the virtual image 70 is limited to provide the more clear
virtual image 70. Therefore, the display quality of the display image 71
can be reliably improved.

[0060] Furthermore, like in the present embodiment, in which the display
image 71 is enlarged by the concave mirror 40, the three-dimensional
deformation of the virtual image 70, which is caused by the curvature of
field, may be enlarged. However, the imaging surface 31 can reduce the
enlarged three-dimensional deformation of the enlarged virtual image 70,
by the adjusting function of the imaging surface 31. Therefore, the
viewer can view the improved virtual image 70, which is enlarged to
enable the easy recognition of the virtual image by the viewer and in
which the shape change and the positional change of the virtual image 70
are limited. Thus, the display quality of the display image 71 can be
further improved.

[0061] Furthermore, in the present embodiment, the reflection surface 41
of the concave mirror 40 is configured to enlarge the display image 71
such that the enlargement (magnification) of the display image 71 in the
horizontal direction (the lateral direction or the first direction) is
larger than enlargement (magnification) of the display image 71 in the
vertical direction (top-to-bottom direction or the second direction). In
such an instance, the deformation of the virtual image 70 in the
horizontal direction caused by the curvature of filed may possibly become
prominent due to the increased enlargement of the virtual image 70 in the
horizontal direction. Therefore, the imaging surface 31 is configured to
be the convex surface, which is curved in the horizontal direction.
Thereby, the adjusting function of the imaging surface 31 discussed above
can effectively reduce the prominent horizontal deformation of the
display image 71. In addition, the imaging surface 31, which has the
simple curved shape that is curved in the horizontal direction, can be
easily formed, so that the screen 30 can be reliably provided. Thereby,
the implementability of the improved display quality of the display image
71 can be increased.

[0062] In addition, according to the present embodiment, the display image
71, which is formed on the imaging surface 31 through the scanning of the
high power laser light, has the high contrast. Thus, the high visibility
of the virtual image 70 can be implemented. As discussed above, the good
display quality can be implemented by the head-up display apparatus 100
of the present embodiment, which enables the viewer to view the virtual
image 70 having the high visibility and the reduced deformation.

[0063] Furthermore, in the present embodiment, due to the combination of
the free-form-surface lens 50 with the laser scanner 10, the display
image 71, which is formed on the imaging surface 31, becomes more clear
with the aid of the focusing point adjusting function of the
free-form-surface lens 50 discussed above. Thus, the viewer can more
easily recognize the virtual image 70 of the display image 71, in which
the deformation is reduced. As a result, when the laser scanner 10 is
combined with the free-from-surface lens 50, the display quality of the
display image 71 can be substantially improved.

[0064] Furthermore, in the present embodiment, the light input surface 51
of the free-form-surface lens 50 has the adjusting function for adjusting
the focusing point of the laser light on the imaging surface 31. The
light input surface 51 is formed as the curved surface, so that the
incident angle (input angle) of the laser light on the light input
surface 51 becomes the angle that is equal to or close to the right
angle. Therefore, the chromatic aberration of the laser light, which
occurs at the time of passing through the free-form-surface lens 50, can
be reduced.

[0065] Furthermore, due to the use of the reflective screen 30, the
portion of the light path of the laser light is bent backward and forward
in the inside of the head-up display apparatus 100. Thus, the size of the
head-up display apparatus 100 can be reduced to enable the installation
of the head-up display apparatus 100 in the instrument panel while
providing the required distance of the light path.

[0066] In the present embodiment, the laser scanner 10 serves as a
projector of the present disclosure. The screen 30 serves as a screen
member of the present disclosure. The concave mirror 40 serves as a
magnifying mirror (also known as a magnifier) of the present disclosure.
The free-form-surface lens 50 servers as a focusing point adjusting
optical system (or the free-form-surface optical element of the focusing
point adjusting optical system) of the present disclosure. The eye box 60
serves as a viewing area of the present disclosure. The windshield 90
serves as a display member of the present disclosure.

[0067] The present disclosure has been described with respect to the above
embodiment. However, the present disclosure is not limited to the above
embodiment, and the above embodiment may be modified within a spirit and
scope of the present disclosure.

[0068] For instance, in the above embodiment, the windshield 90 is used as
the display member of the present disclosure, onto which the display
image 71 is projected at the head-up display apparatus 100. However, the
display member, on which the projection surface is formed, is not limited
to the windshield 90. For example, with reference to FIG. 2, the display
member may be a combiner (separate member) 92 having a projection surface
93. This combiner 92 is made of a light transmissive material and is
placed adjacent to, more specifically attached to the interior surface of
the windshield 90 located inside of the passenger compartment of the
vehicle.

[0069] Furthermore, the combiner 92 may be formed separately or provided
separately from the windshield 90. In the case where the combiner 92 is
used as the display member, the concave mirror, which serves as the
magnifying mirror, may be eliminated. In a case where the projection
surface 93 of the combiner 92 is a curved concave surface, the virtual
image, which is viewed by the driver (viewer), is an enlarged image that
is enlarged from the display image formed on the imaging surface.
Therefore, in such a case, the magnifying mirror can be eliminated.

[0070] Furthermore, in the case where the combiner 92, which is formed
separately from the windshield 90, is used as the display member, the
combiner 92 may be configured into a planar form. In addition, in a case
where the desired magnification can be obtained with the magnifying
mirror, it may not be required to have the enlarging function, which is
achieved by the curvature of the projection surface. Thereby, the case,
in which the display image is projected onto the planar projection
surface, is possible.

[0071] In the above embodiment, the curvature of the reflection surface 41
and the curvature of the projection surface 91 enable the driver (viewer)
to see the virtual image 70 of the display image 71, which is enlarged to
have the larger magnification in the horizontal direction that is larger
than the magnification in the vertical direction. Thereby, the imaging
surface 31 is curved only in the horizontal direction to effectively
limit the deformation of the virtual image 70 in the horizontal direction
to provide the required angle of view in the horizontal direction.
However, the magnification in the horizontal direction and the
magnification in the vertical direction can be appropriately changed
depending on the requirement (or a need). Thus, in the case where the
angle of view is required in the vertical direction, the imaging surface
may be curved in the vertical direction in addition to the horizontal
direction. Specifically, in such a case, it is desirable that the
polynomial (see FIG. 8), which defines the shape of the imaging surface,
includes the even-order term (e.g., the quadratic term) of y.
Furthermore, in such a case, the shape of the imaging surface may
correspond to the corresponding surface contour, i.e., the corresponding
surface curvature of the projection surface 91 in the horizontal
direction and the corresponding surface curvature of the projection
surface 91 in the vertical direction. The above modification is also
applicable to the reflection surface 41 of the concave mirror 40.

[0072] In the above embodiment, the reflective screen 30 is used. However,
as long as the screen is configured to have the convex surface, the
screen is not limited to the reflective type. For example, the screen 30
may be formed as a transmission screen, which is made of a light
transmissive material. In such a case, the laser scanner projects the
laser light, which forms the display image, from the opposite side of the
convex imaging surface of the screen, which is opposite from the concave
mirror.

[0073] In the above embodiment, the imaging surface 31 of the screen 30 is
the curved parabolic surface, which has the quadratic term. However, the
shape of the windshield and the shape of the projection surface may vary
depending on the type of the vehicle, on which the head-up display
apparatus is installed. Therefore, in a case where the windshield and the
projection surface are tilted relative to the vertical direction, it is
desirable that the polynomial (see FIG. 8), which defines the shape of
the imaging surface, includes the odd-order term (e.g., the cubic term)
of y to compensate (correct or limit) the effect of the tilt of the
projection surface.

[0074] In the above embodiment, the free-form-surface lens 50, which is
placed between the laser scanner 10 and the screen 30, serves as the
focusing point adjusting optical system (or the free-form-surface optical
element of the focusing point adjusting optical system) of the present
disclosure. Alternatively, in place of the free-form-surface lens 50, a
free-form-surface mirror may be used as the focusing point adjusting
optical system (or the free-form-surface optical element of the focusing
point adjusting optical system) of the present disclosure. In the case
where the free-form-surface mirror is used, the chromatic aberration,
which is generated in the laser light, can be limited at the time of
adjusting the focusing point of the laser light. Further alternatively,
for example, multiple lenses and/or mirrors may be used to form the
focusing point adjusting optical system of the present disclosure.
Further alternatively, the condenser lens 25 of the laser scanner 10 may
be formed as a part of the focusing point adjusting optical system of the
present disclosure.

[0075] In the above embodiment, the free-form-surface lens 50 has the
light output surface 52, which is formed as the planar surface, and the
light input surface 51, which is formed as the free-form surface.
Alternatively, the light output surface of the free-form-surface lens may
have a light output surface, which is formed as a free-form surface, and
a light input surface, which is formed as a planar surface. Further
alternatively, a light input surface and a light output surface of the
free-form-surface lens may be formed as free-form-surfaces, respectively.
Further alternatively, the free-from-surface lens may have a light input
surface, which is formed as a simple concave surface or a simple convex
surface, and a light output surface, which is formed as a simple concave
surface or a simple convex surface.

[0076] In the above embodiment, as discussed with reference to FIG. 6, the
focusing points of the laser lights, which are initially set to place the
focusing point of the laser light onto the center portion 32 of the
imaging surface 31 while displacing the focusing point of the laser light
away from the edge portion 33 of the imaging surface 31 on the side of
the imaging surface 31 where the laser scanner 10 is located, are
adjusted by the function of the free-form-surface lens 50 such that the
focusing point of the laser light, which is initially displaced away from
the edge portion 33 of the imaging surface 31, is placed onto the edge
portion 33 of the imaging surface 31, so that the display image 71, which
is formed on the imaging surface 31, becomes more clear throughout the
entire range of the display image 71. However, this may be modified in
any other appropriate manner. For example, the focusing points of the
laser lights, which are initially set to place the focusing point of the
laser light onto the edge portion 33 of the imaging surface 31 while
displacing the focusing point of the laser light away from the center
portion 32 of the imaging surface 31 on the side of the imaging surface
31 opposite from the laser scanner 10, may be adjusted by the function of
the free-form-surface lens 50 such that the focusing point of the laser
light, which is initially displaced away from the center portion 32 of
the imaging surface 31, is placed onto the center portion 32 of the
imaging surface 31 by displacing it toward the laser scanner 10, so that
the display image 71, which is formed on the imaging surface 31, becomes
more clear throughout the entire range of the display image 71. Further
alternatively, the focusing points of the laser lights, which are
initially set to place the focusing point of the laser light onto an
intermediate location of the imaging surface 31 between the center
portion 32 and the edge portion 33 while displacing the focusing point of
the laser light away from the center portion 32 and displacing the
focusing point of the laser light away from the edge portion 33, may be
adjusted by the function of the free-form-surface lens 50 such that the
focusing point of the laser light, which is initially displaced away from
the center portion 32, is placed onto the center portion 32 by displacing
it toward the laser scanner 10, and the focusing point of the laser
light, which is initially displaced away from the edge portion 33, is
placed onto the edge portion 33 by displacing it away from the laser
scanner 10, so that the display image 71, which is formed on the imaging
surface 31, becomes more clear throughout the entire range of the display
image 71.

[0077] In the above embodiment, the optical axis of the laser light, which
is transmitted from the laser scanner 10 to the imaging surface 31, is
along the y-z plane. Because of this arrangement, the polynomial (see
FIG. 11), which defines the shape of the light input surface 51 of the
free-form-surface lens 50, includes the odd-order term (specifically, the
cubic term) of y. However, the relative position of the laser scanner
with respect to the imaging surface needs to be changeable in an
appropriate manner depending on the available space in the inside of the
instrument panel and the shape of the windshield of the type of the
vehicle, in which the head-up display apparatus is installed. Therefore,
it is desirable that the polynomial, which defines the shape of the
surface of the free-form-surface lens, includes the term(s), which
corresponds to the installation location of the laser scanner in the
vehicle. Specifically, in a case where the optical axis of the laser
light, which is transmitted from the laser scanner to the imaging
surface, is along an x-z plane (a plane defined by the x-axis and the
z-axis), i.e., in a case where the laser light is projected onto the
projecting surface from a lateral side of the imaging surface, it is
desirable that the polynomial, which defines the shape of the surface of
the free-form-surface lens, includes the odd-order term of x.
Furthermore, in a case where the laser light is projected onto the
projecting surface from a diagonally lower side of the projecting
surface, it is desirable that the polynomial, which defines the shape of
the surface of the free-form-surface lens, includes both of the odd-order
term of x and the odd-order term of y.

[0078] In the above embodiment, the laser scanner 10, which forms the
display image 71 on the imaging surface 31 through the scanning of the
laser light (i.e., the steering of the laser light with the mirror
portion 27), is used as the projector of the present disclosure. However,
various other types of projectors may be used as the projector of the
present disclosure as long as such a projector can project the light,
which forms the display image on the imaging surface. Specifically, for
example, a projector, which includes a liquid crystal on silicon (LCOS)
or a digital mirror device (DMD) together with a light source and an
optical system (e.g., a lens(es)), may be used as a the projector of the
present disclosure.

[0079] The LCOS is formed by holding, i.e., clamping a liquid crystal
layer between a silicon substrate and a light transmissive substrate. The
liquid crystal layer forms a plurality of arrayed pixels. A circuit,
which drives the liquid crystal, and an electrode, which reflects the
light, are provided at the silicon substrate. The light of the light
source, which enters the LCOS through the light transmissive substrate,
passes through the liquid crystal layer and is reflected by the electrode
provided at the silicon substrate, so that the reflected light exits the
LCOS. When an original image, which later becomes the display image, is
formed in the liquid crystal layer, the projector having such an LCOS can
project the light that forms the display image on the imaging surface.

[0080] The DMD is formed by arraying a large number of micro-mirrors on a
substrate. Each of the micro-mirrors forms a corresponding pixel. A tilt
angle of each micro-mirror can be changed based on a control signal. The
light of the light source, which enters the DMD, is reflected by each
micro-mirror. The DMD can form the image by controlling the tilt angle of
each of the micro-mirrors. Thus, the projector, which has the DMD, can
project the light, which forms the display image on the imaging surface.

[0081] In the above embodiment, the MEMS mirror 26, which has the multiple
movable pivots, i.e., the high speed pivots 27a and the low speed pivots
28a, are provided to scan, i.e., steer the laser light. However, the
laser scanner may have a plurality of MEMS mirrors, each of which has a
single movable pivot (or two pivots that extend only in a corresponding
one of the horizontal direction and the vertical direction).
Specifically, a first MEMS mirror, which scans, i.e., steers the laser
light in the horizontal direction, and a second MEMS mirror, which scans,
i.e., steers the laser light in the vertical direction, may be combined
to implement the function of the MEMS mirror 26 of the above embodiment,
which forms the two-dimensional image.

[0082] In the above embodiment, the present disclosure is applied to the
head-up display apparatus, which projects the display image 71 on the
windshield 90 of the vehicle (e.g., the automobile). However, the present
disclosure can be applied to various types of head-up displays, which are
adapted to be installed in various other types of transportation
apparatuses (e.g., other types of vehicles, such as airplanes, ships,
trains) and to enable a viewer to view the virtual image 70 of the
display image 71.

[0083] Additional advantages and modifications will readily occur to those
skilled in the art. The present disclosure in its broader terms is
therefore not limited to the specific details, representative apparatus,
and illustrative examples shown and described.